Depth measurement and depth control or automatic depth control for a hollow to be produced by a laser processing device

ABSTRACT

According to a method for a depth measurement the depths of measuring points on a calibration surface are measured and correction values depending on differences between the measured values and known values are used and stored for a later correction. According to a method for the layer-wise production of a hollow the horizontal boundaries (x g , y g ) for the removal of a layer (S i+1 ) depending on the hollow depth (z) were determined from the shape definition of the hollow. The measured values can be continuously stored and used for a later control of the laser processing device.

The invention, in the most general sense, relates to the depthmeasurement and the depth control or automatic depth control of a hollowto be produced by al laser processing device.

A depth measuring system is known from the DE OS 42 06 499. In this casean incoherent process light is analysed, for example by a triangulationmethod or by analysing the distance between different reflections of thelight spot.

For representing the light spot on a sensor or a sensor array a lens isrequired. Since the light spot may be disposed on any position in theworking area Bx, By, it has to be taken care that focusing issufficiently accurate in all positions of the light spot in the workingarea of the laser processing device.

Conventional lenses have a spherical focal surface. Since the surfacecurrently worked by the laser processing device, however, is generallynoi spherical, a slight defocusing will therefore always occur.So-called Fθ lenses are corrected so that they have a flat focalsurface. However, this focal surface also is not completely plane sothat defocusing may occur. Depending on the desired measuring accuracysuch lack of definition may lead to an unacceptable loss of accuracy.The mentioned Fθ-lenses enable measuring accuracies in the range ofabout 100 sum. The inaccuracy of the focal plane of said Fθ-lenses isalso in this range. In modern laser processing devices, however,production accuracies of a few micrometers are obtainable orcontrollable. Then, however, correspondingly accurate measuring systemsare also required which, in particular, should be about as accurate asthe production accuracy. The required accuracies can not be obtainedwith the measuring system known from the DE 42 06 499. This isparticularly true when the measuring system is integrated in theprocessing system, and especially when both use the same optics. Theprocess light used for the measurement then passes through a comparablywide section of the imaging system so that the mentioned inaccuracieswill clearly occur. They may be in the range of tenths of millimeters.

From the DE 42 09 933 a method for partially changing the surfaces ofmetallic and non-metallic bodies with an Nd:YAG-laser is known. Asubstance removal in the depth direction is not described therein.

The formation of hollows by means of laser processing devices has so farbeen effected by carrying out a layer-wise removal of substance. Thelayer thickness is respectively predetermined and will be controlled.This results in the drawback that performance reserves need to beprovided to ensure that the target control can safely be obtained in anycase. In addition, known methods have the disadvantage that for a depthcontrol the success of the control is effected in another position thanthe measuring position. This is due to the processing speed of thecontroller and the guiding speed of the laser beam. During theprocessing period the laser beam is moved ahead so that the controlresult occurs locally displaced. This tends to apply also to furtherlayers so that difficulties concerning the depth control may occur.

It is the object of the invention to provide a method and an apparatusfor an exact depth measurement and for an accurate depth control inlaser processing devices.

Said object is solved by the features of the independent claims.Dependent claims are related to preferred embodiments of the invention.

An exact depth measurement can be obtained by calibrating the actualsensor system particularly for the optics used. To this end a knowncalibration surface, preferably a plane, is measured. The actual valueswill then be compared with the known values, and correction values forthe respective position in the working area will be generated and storedin accordance with the difference.

In the present description it is assumed that the depth of the hollowextends in the z-direction of a rectangular coordinate system while theworking area is a plane extending substantially in the x-y-plane of thecoordinate system (see FIG. 1). For the working area Bx, By thus a twodimensional correction field is determined which may then be used forthe correction of the actual measurements.

Instead of working with fixed layer thicknesses like in the case of thestate of the art, it is also possible to determine the current depth zof the hollow and to determine the boundaries in the x- and y-directionsfor a following, particularly the next layer to be removed, from thedefinition of the shape to be produced depending on said absolute hollowdepth. In the case of a form converging in tub-shape towards the bottomit might, for example, be found that during the removal of one layer thematerial was penetrated deeper than intended. In the next layer thennarrower boundaries would be set in the x- and y-directions.

A further enhancement of the accuracy will be obtained when not only theabsolute depth of the hollow is taken into consideration in calculatingthe boundaries in the horizontal direction for the next layer but alsothe layer thickness currently removed with the currently usedparameters. With said layer thickness Δz a more accurate progress in thez-direction of the shape definition is achievable so that,correspondingly, more accurate boundaries may be calculated for the nextlayer.

To obtain “a priori” information for the control of the apparatus thecontinuously determined depth measurement values may be stored,particularly in accordance with their x- and y-coordinate values. The sostored information may be used in the further progress to takeappropriate measures.

It is to be explicitly noted here that the applicant has filed anotherapplication relating to a method and apparatus for processing aworkpiece with a laser at a date very close to the application date ofthe present patent application, namely U.S. Ser. No. 09/186,353, filedMay 18, 2001. Herein and if required below in the following text, saidapplication is explicitly referred to.

Below individual embodiments of the invention are described withreference to the appended drawings in which:

FIG. 1 is a schematic view of a laser processing device;

FIG. 2 is a schematic functional block diagram of a controller;

FIG. 3 shows the depth measuring device of FIG. 2;

FIG. 4 shows the control unit of FIG. 2 for determining the substanceremoval boundaries in one layer;

FIG. 5 shows the control unit of FIG. 2 for storing measured values; and

FIGS. 6 and 7 are a plan view and a cross sectional view schematicallyshowing a workpiece for discussing general considerations.

FIG. 1 is a schematic view of a laser processing device. If required thedescription will be given with reference to the mentioned rectangularx-y-z-coordinate system, x and z being shown in the plane of the drawingand y extending downwards through the plane of the drawing.

A column 16 carries a working head 13 and a workpiece table 14 which isshiftable if required. In general a relative movement betveen the head13 and the workpiece 11 is possible at least in the x-y-plane. This isindicated by rollers 15 between the workpiece table 14 and the foot 16.Instead or in addition the head 13 may also be movable. In the workpiece11 a hollow 10 is formed. The hollow is generated by means of a laserbeam 12. As a rule, a layer-wise removal of substance is carried out bysuccessively removing layers respectively located in different positionsin the z-direction and extending in the x-y-plane from the top to thebottom. In FIG. 6 this is schematically shown: In the upper part of thecross sectional view a line 107 shows the desired final shape togetherwith the visible contours. Said final shape is generated by a layer-wiseremoval of substance. The layers are indicated in the illustration 106.The broken lines show layers already removed while the continuous linesindicate layers yet to be processed. The currently processed layer isdenoted by S_(i), the previous layer by S_(i−1), the following layers byS_(i+). Combinations of the mentioned possibilities are also possible.

For removing a layer different strategies are feasible: Within theworking area Bx, By of the head the laser beam is guided across thesurface by a suitable laser beam guidance. Meandering embodiments areshown. In the upper part of the plan view an embodiment is shown inwhich the beam guidance principally scans the whole working area Bx, By,the laser beam being turned on only when it scans a surface to beprocessed. i.e. the bottom of the hollow 10. That corresponds to thecontinuous lines 101 b, while the broken lines IOIa indicate the “darkpath”. In the lower part of the plan view, on the other hand, anembodiment is shown in which the laser beam guidance guides the laseronly across the surfaces to be processed, i.e. across the current bottomof the hollow. When a layer S_(i) is removed, the process is continuedwith the removal of the next layer S_(i)+I.

The working area Bx, By is generally limited by constructionalconditions. In general rectangular areas are concerned outside of whichthe laser beam can no longer be guided. In the lower part of FIG. 1 thisis schematically shown. Here the working head is assumed to be a spotlight source 13. The deflection of the beam may be effected between afarthest possible left position 12 a and a farthest possible rightposition 12 b. Therewith an area Bx in the x-direction is obtained. Thesame applies analogously to the y-coordinate.

The apparatus of FIG. 1 is provided with a controller 17 which isconnected to the processing device via lines 18. The control unit 17(which will simply be referred to as controller below) may be compact orformed spatially distributed. It will generally comprise digitalcomponents, for example a process computer.

FIG. 2 schematically shows a functional block diagram of theconstruction of the controller 17. N signal input lines 18 a and msignal output lines 18 b are provided. They passdriver/coupling/converter/processing components 67 a, 67 b which carryout conversions related to data formats, performance and the likes. Thecontroller 17 comprises at least one memory 64 in which different kindsof data can be stored. In addition, different general control orautomatic control functions 65 are provided, for example for laserscanning, laser beam guidance, etc.). 68 denoted functions correspondingto the functions and features described in U.S. Ser. No. 09/806,353mentioned above. They may be provided together with the functionsaccording to the invention and may have advantageous effects. 66 denotesa channel enabling the required communication between the individualones. As far as it is to be regarded as hardware it may, for example, bea bus of a computer.

61 denotes the function of a depth measurement according to theinvention, 62 denotes a control function for determining the processingboundaries in a layer S_(i) according to the invention, and 63 denotes afunction for storing and later analysing the measured values accordingto the invention. The functions 61-63 operate with at least the memory64 and with other functions depending on the requirements. They may alsointeract with the functions 68 described in the two other applications.

FIG. 3 shows an exemplary embodiment of a depth measuring device.Numerals corresponding to the ones used in the previous drawings denoteidentical components. An embodiment is shown in which the primary sensor70 is spatially integrated with the working head 13 (for emitting theprocessing laser beam). In particular the process light analysed by thesensor 70 at least partially passes the same optics as the working laserbeam. A line sensor is shown which receives an image of the light spoton the working position on the workpiece 11 just illuminated by thelaser beam. The measuring principle of the sensor may be as described inthe DE OS 42 06 499. The sensor outputs a more or less widely spreadsignal which is received by the controller 17 and particularly by thedepth measuring unit 61 according to the invention. 71 denotes a complexsignal processing unit which transforms a comparably crude sensor signalinto a depth value z (along the z-coordinate in FIG. 1).

To obtain an exact measurement the depth measuring device is calibratedprevious to the actual depth measurement. To this end a calibratingsurface is measured. The calibrating surface has a known shape which ispreferably flat. Preferably the calibrating surface is so large that thewhole working area Bx, By can be accommodated on it. In one calibratingpass the height of the calibrating surface in the z-direction ismeasured on different points (distributed, for example, in a grid shape)in the working area Bx, By. The so obtained measured value will becompared with the known height of the calibrating surface (denoted by72) in a comparator 73. The difference provides a scale for themeasuring error. The difference may be stored in the memory 74 dependingon the position or may be used for determining a correction value to bestored depending on the position as well. “Depending on the position” inthis connection means depending on the position in the x- ory-directions within the working area Bx, By. The x- and y-coordinatesare known to the controller 17 from the general functions 65.

During the calibrating process thus a plane correction field is storedwhich may than be used for correcting the actually measured values. Thisis symbolised by a component 75. It receives an actually measured valuevia the sensor 70, lines 18, 66 and a signal former 71. In addition, itreceives a correction value corresponding to the depth measurementposition in its position from the memory 74. In the correction device 75the measured value is corrected and output or held for other systemfunctions. The correction may be carried out by addition and/or bymultiplication. An identification field may also be provided. Acorrection depending on the absolute depth z may also be provided.

During the calibration process the calibration surface may be measured aplurality of times while being shifted in the horizontal direction (xand/or y) between the individual measurements. In this case correctionvalues depending on the measured values obtained for the respectiveposition x, y in the working area Bx, By will be determined for theindividual positions in the working area Bx, By (by averaging,interpolation or the likes). For the correction of actually measuredvalues also interpolations or averaging can be carried out, particularlywhen no or only a remotely located correction value exists for thecurrent measuring position.

The calibration according to the invention or the depth measurementaccording to the invention enable a measuring accuracy in the range of afew micrometers, preferably below 1 μm. The correction values may, asfar they are correction values obtained by addition, correspond to avalue of up to 1 mm or more.

The depth measurement in the z-direction described above may but neednot be used in the functions described below.

FIG. 4 schematically shows a controller for the layer removal. The basicconsiderations will be explained with reference to FIG. 7. Identicalnumerals therein denote features corresponding to the ones in theprevious figures. FIG. 7 shows the laser beam 7 impinging on the bottom112 of the hollow 10. A moving direction of the laser beam 12 in thedirection of the arrow 111 (i.e. in the x-direction in this case)effected by the laser beam guidance is assumed. The material of thelayer S_(i) is vaporised and liquefied and thus removed. This isindicated by the arrows directed away from the working position 110. Thethickness of a layer is assumed to be Δz, the measured absolute depth isz. The wall 113 of the hollow 10 is to follow the contour 107 in thelower layers as well. The boundary xg for the removal in the followinglayer S_(i+1) therefore depends on the depth z particularly in the caseof inclined walls, a dz will lead to a dxg. As long as it is possible toset the depth z to predetermined values from layer to layer theboundaries for the layer removal in a layer xg (and correspondingly yg)can also be previously set and then adjusted. That corresponds to afixed programming of the device. It may, however, be desirable not toinsist in said layer thicknesses. Sometimes it may also be technicallyimpossible.

It will then be advantageous to determine the removal boundaries in thex-y-plane for the next layer S_(i+1) based on the actual depth z since achange of z will also result in a change of xg and yg. This correspondsto a flexible programming. A device for realising that consideration isschematically shown in FIG. 4. It comprises a control device 81 fordetermining the horizontal boundaries xg, yg for the substance removalin a following layer, particularly S_(i+1) from the shape definitionstored in a memory 83 depending of the depth z of the hollow. To thisend, on the one hand, the control unit 81 receives data containing theshape definition, and, on the other hand, the depth z (or a valuederived from it, for example, filtered or averaged). From said data theboundaries xg, yg in the horizontal direction for the layer removal maybe determined and supplied to the conventional components 65 foradjusting said values.

A further increase of the accuracy will be obtained when not only theabsolute depth z but also the layer thickness Δz just removed with thecurrent parameters is considered for determining the substance removalboundaries xg, yg. The calculation “into the depth of the hollow” doesthen not need to be carried out using a theoretical value for the layerthickness, but the currently actually removed layer thickness may beused.

When only the measured depth z (together with a theoretical value forthe layer thickness) is taken into consideration for the boundarydetermination, the generation of a cumulative error is avoided and atmost a non-cumulative error corresponding to the difference between thetheoretical and the actual layer thickness occurs which may be tolerablein some cases. When the actual layer thickness Δz is also considered inthe determination of the boundaries the occurance of said residual errorwill also be prevented.

FIG. 4 shows means 82 for determining the layer thickness. It may, forexample, be designed so that it will remember measured values z of theprevious layer S_(i−1) and then compare these with the values measuredduring the removal of the layer S_(i). The difference corresponds to thelayer thickness Δz. In this case also filtered or averaged values may beused.

The definition of the shape of the hollow may, for example, be stored inthe memory 83 in the form of CAD-data. The device 81 is possibly arelatively complex structure which can calculate intersection edgesbetween a plane (corresponding to a value of z+Δz) and a shape(corresponding to the shape definition of the hollow) from the kind ofdata stored in the memory 83.

FIG. 5 shows a function for continuously storing the continuouslymeasured depth data z. The storage is preferably carried out in memorylocations corresponding to the position of the measured location in theworking area of the device. It is not always possible to produce thecurrent bottom 112 so evenly as shown in FIG. 7. Rather, waviness orplateau or indentations may occur. In FIG. 6 the numeral 103 denotes aplateau under consideration the movement of the laser beam 12 in FIG. 7in the direction of the arrow 111 a feeding speed v_(x) may bedetermined, if, on the other hand, it is assumed that the reaction speedof the system is limited to one measurement, a time t_(R) can bedetermined as a reaction period which will pass until a measured valueof z can have an effect on the laser 12. Due to the reaction time t_(R)and the feeding speed v_(x) control interventions will principallybecome effective with a spatial offset. That corresponds to a dead timeunder a control technical aspect. In disadvantageous cases oscillations(waviness) may occur. The offset corresponds to Δx=v_(x)·t_(R)

When the depth z is measured continuously or quasi-continuously it mayalso be continuously written into a memory 91 and used in a suitablemanner later. A topography or topographic mapping of the current hollowbottom will then be generated in the memory 91, said mapping being atabular reflection of the respectively measured depth values z. Thedensity of the measuring points on the surface towards high values islimited in the feeding direction of the laser beam by the feeding speedv_(x) and the reaction time t_(R) and can be selected below said limits.In the case of a meandering surface coverage according to FIG. 7 thedensity of the density of the measuring points is determined by thetrack distance of the meanders in the direction transverse to thefeeding direction.

When the topographical mapping or the topography indicates, for example,a plateau 103, this a priori information may be used for levelling outsaid irregularL ity without the temporary offset due to the reactiontime of the system preventing the error correction. Owing to the apriory information the interaction parameters of the laser can bechanged (towards a stronger substance removal in the case of plateaux,towards a weaker substance removal in the case of indentations) in therange of a recognised irregularity, or additional layers for removingonly the irregularity (the plateau or the land around the indentation)may be inserted in the case of larger deviations.

A change of the interaction parameters of the laser beam may be effectedwhen the laser beam passes the vicinity of the error again within thesame layer (for example in the neighbouring track in the case of ameandering guidance according to FIG. 6). It is assumed here that theeffect of the laser beam can not be accurately limited to one track. Theeffective area is rather indistinctly limited so that the laser beamincident on the current hollow bottom will not only be effective in the“ideal”, currently observed track but also in the adjacent tracks. Inaddition the interaction parameters of the laser beam may also bechanged in the following layers, for example in the next lower layer, tolevel out an irregularity recognised in an earlier layer.

With the described adjustment of the interaction parameters depending onthe stored hollow depth data as a control the control of the laserdepending on the currently measured values may be maintained. The laser,however, may also be operated depending on the currently measured valueswithout said control so that it is controlled only depending on thestored parameters.

The topographic mapping described may advantageously be combined withthe determination of the substance removal boundaries in the horizontaldirection described with reference to FIG. 4. Said techniques may beused in connection with the described measuring arrangement (referencinga calibrating curve) individually or in combination. The topographicmapping method described with reference to FIG. 5 can also be usedtogether with the method for adjusting the relative position describedin the other application Ser. No. 09/806,353 of the applicant. Forexample, the relative positions of the working head and the workpiecemay be selected so that critical areas on the workpiece (for example aplateau 103 or an indentation) will not come to be located in theboundary sections of the working area of the device so that a reliableprocessing of the corresponding position will become possible.

1. Apparatus for making a specific shaped hollow (10) in a work piece(11), comprising a laser machining apparatus (12-18) which is configuredto, in a layer-wise manner, remove material of a work piece (11) inhorizontal layers (S; xy) corresponding to the specific shape, and ameasurement apparatus (70-73) which is configured to continuouslymeasure the depth (z) of the hollow, characterized by a controlapparatus (81) which is configured to determine the boundaries (x_(g),y_(g)) in horizontal direction for removal in a subsequent layer(S_(i+1)) in accordance with the depth (z) of the hollow from the formdefinition.
 2. Apparatus according to claim 1, characterized in that thecontrol apparatus comprises a determining means (82) for determining thethickness (Dz) of a removed layer (S_(l)) from the measured depth of thehollow, and the control apparatus (81) determines the boundaries (x_(g),Y_(g)) in horizontal direction for removal in a subsequent layer(S_(i+1)) also in accordance with the determined layer thickness (Dz).3. Apparatus according to claim 1, characterized by a memory (83) forstoring the form definition of the hollow (10).
 4. Apparatus for makinga specifically shaped hollow (10) in a work piece (11), comprising alaser machining apparatus (12-18), which is configured to, in alayer-wise manner, remove material from the work piece (11)corresponding to the specific shape, and a measurement apparatus whichis configured to continuously measure the depth (z) of the hollow,characterized by a memory means (91) which is configured to continuouslystore the measurement values together with the respective coordinates orat memory locations corresponding to the respective coordinates, and acontrol apparatus (63, 92, 93) which is configured to control the lasermachining apparatus (12-18) in accordance with the stored measurementvalues.
 5. Apparatus according to claim 4, characterized in that thecontrol apparatus uses a stored measurement value if within the samelayer the laser beam is close by a site corresponding to saidmeasurement value, and/or if, in a deeper layer, the laser is close byor at a site corresponding to the measurement value.
 6. Apparatusaccording to claim 4, characterized in that the control apparatus uses ameasurement value for the instantaneous or later adjustment of theinteraction parameters of the laser beam.
 7. Apparatus according toclaim 1, the laser machining apparatus (12-18) guiding, by means of alaser beam guidance, the laser light across the surface of a work piecewithin a working area defined by the apparatus, comprising a depthsensor (70, 71) which uses for depth measurement light emanating fromthe working site and generates a measurement value, characterized by acalibrating apparatus (72-74) adapted to measure a preferably flatcalibrating surface and having a memory (73) for storing correctionvalues in accordance with differences between measurement values andknown values together with the respective coordinates or at memorylocations corresponding to the respective coordinates, and a correctionapparatus (74, 75) which corrects the measurement value in accordancewith the position of the site with reference to the correction valuesstored in said memory (74).
 8. Apparatus according to claim 7,characterized in that the correction is made by adding a value and/or bymultiplying a value.
 9. Apparatus according to claim 7, characterized inaccordance with the depth of the hollow.